My son is 5 months old. For the last 2 weeks or so he has been very cranky when eating and being burped. He is fine when given solids, but when given a bottle he arches his back after a few swallows and starts to cry as if he is in pain. When I burp him the same occurs, only when he finally burps he spitts up each time the formula he just finish drinking. Could this be acid reflux? Should I be concerned?
I was planning on giving him Enfamil AR During each feeding instead of regullar Enfamil Lipid, but I am afraid that he will get constipated.
FYI: Every three to four hours I feed him in this order every day.
Two tbsp of cereal in the morning with a spoon following 3 ounces of formula.
4 ounces of formula
2 ounces of formula with 2 tsp of vegetables and 2 tsp of dessert mixed inside formula
2 tbsp of cereal with a spoon following 3 ounces of formula
5 ounces of Enfamil AR prior to putting him to sleep (around 8:30pm)
During the day his discomfort seems to be the worst. Once in a while it affects him at night. He rarely ever cries, so I do not know what to do, please help me.

It's seems likely to be reflux.
You may consider thickening his feeds with rice cereal (one table-spoon of cereal for every ounce of formula).
You can try this and tell us what happens.
If there is no improvement, you may need to change the formula or even give medication, but let's wait and see.